The present invention relates to an apparatus for and a method of scrambling and decoding a television signal in a secure television system.
The scrambling of television signals and the associated decoding thereof has been used and extensively described. The need for private transmission particularly arises in the cable television field where several of the many available television channels may carry programs which may only be viewed under special conditions, such as after making prepayments.
Many devices have been proposed whereby television signals are transmitted in a scrambled form and cannot be displayed by a conventional receiver unless a special decoding device is employed. Generally, the scrambling technique requires complex decoders at the subscribers and present a relatively high investment when a large number of subscribers are to be provided with decoders to unscramble the received television signal.
Secret signaling is old in the art. An early U.S. patent to Mathes U.S. Pat. No. 1,542,566 issued in 1925 and described a system wherein the frequency band occupied by the signal was subdivided by filters into discrete separate bands. The subdivided bands are shifted in phase relative to each other and then recombined for transmission. At the receiver, the individual sub-bands are phase shifted in the order necessary to reconstruct the original signal.
The U.S. patent to Nyguist et al., U.S. Pat. No. 1,726,578 teaches a secret signaling system wherein different frequencies in a signal are subjected to different degrees of delay. The original signal is restored by passing the received scrambled signal through a network which is complementary to that employed at the transmitter.
In the U.S. patent to Handsell U.S. Pat. No. 2,169,357 a secret signaling device is disclosed wherein a carrier is phase modulated and subsequently modulated with an audio signal to be kept private. The audio signal is recovered by re-introducing the carrier followed by filtering to remove higher frequency beats produced with the re-introduced carrier.
In the U.S. patent to Clothier U.S. Pat. No. 2,678,347 a system is described for rendering a television signal private except to the subscriber who is provided with a specific decoding mechanism. An error signal is introduced at the transmitter to phase modulate the video signal and this error signal is removed at the subscriber receiver. The U.S. patent to Bartelink U.S. Pat. No. 2,833,850 teaches another private subscriber television system wherein the video signal is scrambled to provide private transmission and reception. Many other television scrambling systems have been proposed such as described in the U.S. patents to Wendt et al., U.S. Pat. No. 2,875,270, Druz et al., U.S. Pat. No. 2,987,576, Kahn U.S. Pat. No. 3,333,052, Zopf et al., U.S. Pat. No. 3,717,206, Court U.S. Pat. No. 3,729,576 and Court U.S. Pat. No. 3,852,519.
In the first Court patent a system is described wherein a video modulated carrier is further modulated with a sine wave in sychronism with the horizontal line frequency. The sine wave's frequency and phase are selected to depress sync pulses and blanking information while enhancing other parts of the video signal. The decoding device at the receiver provides a decoding sine wave which is in antiphase with the encoding sine wave modulation.
In the second Court patent, an rf modulated television signal has its normal, unscrambled carrier replaced with an amplitude adjusted scrambling carrier of the same frequency but different phase. The resulting television signal is effectively scrambled and rendered private by virtue of the impairment of the displayability of the scrambled television signal on a conventional television set.